FIG. 1 illustrates digital image acquisition apparatus, for example a camera phone. The apparatus 10 comprises an Image Signal Processor, ISP, 14, which is in general, a general purpose CPU with relatively limited processing power. Typically, the ISP 14 is a dedicated chip or chip-set with a sensor interface 20 having dedicated hardware units that facilitate image processing including image pipeline 22. Images acquired by an imaging sensor 16 are provided to the ISP 14 through the sensor interface 20.
The apparatus further comprises a relatively powerful host processor 12, for example, an ARM9, which is arranged to receive an image stream from the ISP 14.
The apparatus 10 is equipped with a display 18, such as an LCD, for displaying preview images, as well as any main image acquired by the apparatus. Preview images are generated automatically once the apparatus is switched on or only in a pre-capture mode in response to half pressing a shutter button. A main image is typically acquired by fully depressing the shutter button.
Conventionally, high level image processing, such as face tracking, is run on the host processor 12 which provides feedback to the pipeline 22 of the ISP 14. The ISP 14 then renders, adjusts and processes subsequent image(s) in the image stream based on the feedback provided by the host processor 12, typically through an I2C interface 24. Thus, acquisition parameters of the subsequent image in the stream may be adjusted such that the image displayed to the user is enhanced.
Such acquisition parameters include focus, exposure and white balance.
Focus determines distinctness or clarity of an image or relevant portion of an image and is dependent on a focal length of a lens and a capture area of the imaging sensor 16. Methods of determining whether an image is in-focus are well known in the art. For example, if a face region is detected in an image, then given that most faces are approximately the same size and the size of the face within an acquired image, an appropriate focal length can be chosen for a subsequent image to ensure the face will appear in focus in the image. Other methods can be based on the overall level of sharpness of an image or portion of an image, for example, as indicated by the values of high frequency DCT coefficients in the image. When these are highest in the image or a region of interest, say a face region, the image can be assumed to be in-focus. Thus, by adjusting the focal length of the lens to maximize sharpness, the focus of an image may be enhanced.
Exposure of an image relates to an amount of light falling on the imaging sensor 16 during acquisition of an image. Thus an under-exposed image appears quite dark and has an overall low luminance level, whereas an overexposed image appears quite bright and has an overall high luminance level. Shutter speed and lens aperture affect the exposure of an image and can therefore be adjusted to improve image quality and the processing of an image. For example, it is well known that face detection and recognition are sensitive to over or under exposure of an image and so exposure can be adjusted to optimize the detection of faces within an image stream.
Due to the fact that most light sources are not 100% pure white, objects illuminated by a light source will be subjected to a colour cast. For example, a halogen light source illuminating a white object will cause the object to appear yellow. In order for a digital image acquisition apparatus to compensate for the colour cast, i.e. perform white balance, it requires a white reference point. Thus, by identifying a point in an image that should be white, for example the sclera of an eye, all other colours in the image may be compensated accordingly. This compensation information may then be utilised to determine the type of illumination under which an image should be acquired.
While adjusting acquisition parameters such as those described above is useful and can improve image quality and processing, the feedback loop to the ISP 14 is relatively slow, thereby causing delays in providing the ISP 14 with the relevant information to rectify the focus, exposure and white balance of an image. This can mean that in a fast changing scene, adjustment indications provided by the host processor 12 may be inappropriate when they are made by the ISP 14 to subsequent images of the stream. Furthermore, typically most of the processing power available to the host processor 12 is required to run the face tracker application, leaving minimal processing power available for carrying out value added processing.
It is desired to have an improved method of face tracking in a digital image acquisition device.